An Update on Apple’s A7: It's Better Than I Thought

When I reviewed the iPhone 5s I didn’t have much time to go in and do the sort of in-depth investigation into Cyclone (Apple’s 64-bit custom ARMv8 core) as I did with Swift (Apple’s custom ARMv7 core from A6) the year before. I had heard rumors that Cyclone was substantially wider than its predecessor but I didn’t really have any proof other than hearsay so I left it out of the article. Instead I surmised in the 5s review that the A7 was likely an evolved Swift core rather than a brand new design, after all - what sense would it make to design a new CPU core and then do it all over again for the next one? It turns out I was quite wrong.

Armed with a bit of custom code and a bunch of low level tests I think I have a far better idea of what Apple’s A7 and Cyclone cores look like now than I did a month ago. I’m still toying with the idea of doing a much deeper investigation into A7, but I wanted to share some of my findings here.

The first task is to understand the width of the machine. With Swift I got lucky in that Apple had left a bunch of public LLVM documentation uncensored, referring to Swift’s 3-wide design. It turns out that although the design might be capable of decoding, issuing and retiring up to three instructions per clock, in most cases it behaved like a 2-wide machine. Mix FP and integer code and you’re looking at a machine that’s more like 1.5 instructions wide. Obviously Swift did very well in the market and its competitors at the time, including Qualcomm’s Krait 300, were similarly capable.

With Cyclone Apple is in a completely different league. As far as I can tell, peak issue width of Cyclone is 6 instructions. That’s at least 2x the width of Swift and Krait, and at best more than 3x the width depending on instruction mix. Limitations on co-issuing FP and integer math have also been lifted as you can run up to four integer adds and two FP adds in parallel. You can also perform up to two loads or stores per clock.

I don’t yet have a good understanding of the number of execution ports and how they’re mapped, but Cyclone appears to be the widest ARM architecture we’ve ever seen at this point. I’m talking wider than Qualcomm’s Krait 400 and even ARM’s Cortex A15.

I did have some low level analysis in the 5s review, where I pointed out the significantly reduced memory latency and increased bandwidth to the A7. It turns out that I was missing a big part of the story back then as well…

A Large System Wide Cache

In our iPhone 5s review I pointed out that the A7 now featured more computational GPU power than the 4th generation iPad. For a device running at 1/8 the resolution of the iPad, the A7’s GPU either meant that Apple had an application that needed tons of GPU performance or it planned on using the A7 in other, higher resolution devices. I speculated it would be the latter, and it turns out that’s indeed the case. For the first time since the iPad 2, Apple once again shares common silicon between the iPhone 5s, iPad Air and iPad mini with Retina Display.

As Brian found out in his investigation after the iPad event last week all three devices use the exact same silicon with the exact same internal model number: S5L8960X. There are no extra cores, no change in GPU configuration and the biggest one: no increase in memory bandwidth.

Previously both the A5X and A6X featured a 128-bit wide memory interface, with half of it seemingly reserved for GPU use exclusively. The non-X parts by comparison only had a 64-bit wide memory interface. The assumption was that a move to such a high resolution display demanded a substantial increase in memory bandwidth. With the A7, Apple takes a step back in memory interface width - so is it enough to hamper the performance of the iPad Air with its 2048 x 1536 display?

The numbers alone tell us the answer is no. In all available graphics benchmarks the iPad Air delivers better performance at its native resolution than the outgoing 4th generation iPad (as you'll soon see). Now many of these benchmarks are bound more by GPU compute rather than memory bandwidth, a side effect of the relative lack of memory bandwidth on modern day mobile platforms. Across the board though I couldn’t find a situation where anything was smoother on the iPad 4 than the iPad Air.

There’s another part of this story. Something I missed in my original A7 analysis. When Chipworks posted a shot of the A7 die many of you correctly identified what appeared to be a 4MB SRAM on the die itself. It's highlighted on the right in the floorplan diagram below:

While I originally assumed that this SRAM might be reserved for use by the ISP, it turns out that it can do a lot more than that. If we look at memory latency (from the perspective of a single CPU core) vs. transfer size on A7 we notice a very interesting phenomenon between 1MB and 4MB:

That SRAM is indeed some sort of a cache before you get to main memory. It’s not the fastest thing in the world, but it’s appreciably quicker than going all the way out to main memory. Available bandwidth is also pretty good:

We’re only looking at bandwidth seen by a single CPU core, but even then we’re talking about 10GB/s. Lookups in this third level cache don’t happen in parallel with main memory requests, so the impact on worst case memory latency is additive unfortunately (a tradeoff of speed vs. power).

I don’t yet have the tools needed to measure the impact of this on-die memory on GPU accesses, but in the worst case scenario it’ll help free up more of the memory interface for use by the GPU. It’s more likely that some graphics requests are cached here as well, with intelligent allocation of bandwidth depending on what type of application you’re running.

That’s the other aspect of what makes A7 so very interesting. This is the first Apple SoC that’s able to deliver good amounts of memory bandwidth to all consumers. A single CPU core can use up 8GB/s of bandwidth. I’m still vetting other SoCs, but so far I haven’t come across anyone in the ARM camp that can compete with what Apple has built here. Only Intel is competitive.

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444 Comments

Yes, it is a deal breaker for me. I'm using iPad 4 with 1GB RAM and it keeps getting low memory. When I open several tabs in Safari (5-6 tabs), when switching among the tabs Safari will refresh the content which will cost data bandwidth if on 4G. And the thing is they may release another iPad Air with 2GB RAM in 6 months.Reply

I had the original iPad, the iPad 2 and the iPad 3 and now the Air. I will completely agree with the review - in your hand this is a HUGE upgrade over the 3. On paper it's not overly impressive, but it's something you have to hold to really appreciate it. I was on the fence about getting the Air but decided to go ahead and get one today after reading this review last night. I'm very impressed with the Air - I'd say got and see one yourself and then see what you think.

Also the iPad 2 might have been the biggest upgrade - the speed difference between that and the first was greatly needed. By the time I got rid of my original iPad for an iPad 2 I was ready to throw the thing through a window because it just seemed so laggy. The iPad 3 obviously had the Retina screen, but basically is an iPad 2 in terms of performance. That's my opinion anyways.Reply

So now it runs Java and Flash without problems and can have other browsers installed that are not based on the safari rotkit. Support for more codecs, IR blaster for remote control, USB and NFC support and ....... or did they just make a thinner faster product with the same functionality as all the other devices? Reply

Hello everyone, new here, sorry for the long (first) post and my english.

I just wanted to say: I just bought the iPad air this morning after returning my nexus 7.2 for having a horrible aspect ratio, google asking for all my information in every app and just general lagginess.Don't get me wrong, the device itself is fast, but there is a lot of input lag, and little freezes all the time when using chrome or just when navigating the play store. I had gotten it because it was definitely cheaper, but the device even crashed a few times in less than two days, and coming from the iPad Mini (which I absolutely loved but gave my girlfriend), this just won't do.

So now I figure, let's just shell out the extra cash and get the new iPad Air. I get the 16Gb "space gray", which looks much, much better on the iPhone, but still a beautifull device.I fire it up, play a bit with it and boom, (first-world) problems.

There is a lot of screen input lag. I can't say precisely but definitely at least 100ms.It's still pretty fast, but using the tablet mainly for drawing and internet browsing, it really bothers me on such an expensive device.

My question is: I really didn't feel input lag to be that slow on the iPad Mini, and I don't know if it's due to the much higher resolution, or just iOS 7 itself, buit does anyone feel it too, and do you think it could be adressed by a future software update? Because i'm thinking of returning the device and just get another iPad Mini without retina display. I don't reaaally need the extra power and resolution, and size/weight is really what's more important to me. That and speed (not power if it makes sense)

As for lag, I haven't experienced any though I don't do much drawing. However, there is not a faster ARM tablet on the market and iOS has been measured as having much lower screen lag than Android.Reply

I think Apple did a great job of cutting the weight down. Unfortunately, no matter how good their hardware is it's strangled by the overly simplistic iOS. I wish I could run Android or Win 8 on the iPad hardware.Reply